This invention relates generally to metal halide arc lamps and in particular, to an electrode for use in miniature and subminiature arc lamps that operate at or below 40 watts.
A low wattage metal halide arc lamp having extremely high efficacy is disclosed in U.S. Pat. No. 5,144,201. The lamp described in this patent has a geometry that is specifically designed to minimize heat loss from the arc chamber thus maintaining the fill gases at a high operating temperature needed to produce high output illumination. A further U.S. Pat. No. 5,083,059 teaches of the desirability of shaping the electrodes used in a miniature or subminiature lamp to further control the loss of heat from the arc chamber.
The arc lamp described in the '201 patent generally includes a glass or quartz envelope having a hollow arc chamber and a pair of opposed outwardly disposed shanks. As taught in the '059 patent, the electrodes each have a relatively large diameter post section and a smaller diameter lead-in section. In assembly, the lead-in section of each electrode is sealed within one of the lamp shanks and the post sections of the electrodes are suspended in the arc chamber in a spaced apart relationship to maintain an arc therebetween. As explained in this patent, the electrode sections are accurately sized to control the flow of energy out of the arc chamber.
The lead-in and post sections of the electrodes have heretofore been individually formed to a desired diameter and then coaxially joined by means of a welding process. Because of the small size of the electrode sections, the welding process is difficult to perform and the weld tends to disrupt the geometry of the electrode in the weld zone and thus adversely effects the ability to control flow of heat out of the arc chamber.
Attempts to mechanically shape or form electrodes used in miniature and subminiature lamps to different diameters have been less than satisfactory simply because the extremely small size sections do not readily lend themselves to mechanical working.
In U.S. Pat. No. 4,452,677 there is disclosed an electro-mechanical process for etching a tungsten electrode to improve the electrode's ability to form an air tight seal with the shank of a quartz envelope. An electrode having a uniform cross sectional area along its entire length is completely immersed in an aqueous solution of NANO.sub.2 and a current then applied to the electrodes for a short period of time to etch the flow lines created by the fabrication process. This, in turn, establishes roughened sites along the outer surface of the electrode which enhances its ability to form a positive seal with the shank of the glass envelope.